Thousands of surface temperature stations around the world have provided millions of temperature records that, over the last hundred years, show rising global temperatures. Since 1979, rising temperatures have been confirmed by satellite readings. Even without direct measurements, natural indicators like melting glaciers, rising sea levels, shifting tree lines toward the poles, and early spring arrivals also indicate that the world is warming.

A: No. 1998 was exceptionally warm, but temperature trends have continued to rise since that time.

Natural climate influences, such as El Nino and its cousin La Nina, can either reinforce or oppose the warming effect from greenhouse gas additions to the atmosphere. During 1998, a particularly strong El Nino reinforced the warming trend and drove temperatures to an exceptionally high level. In the absence of such strong reinforcement, subsequent years were cooler, but the longer term temperature trend has still been upward. This rise has continued to the point where both 2005 and 2010 are considered by some analyses to be warmer than 1998 without the reinforcement of an El Nino effect as strong as that seen in 1998.

Q: In the 1970s, wasn’t the scientific community warning of a coming ice age?

A: While a small number of studies focused on cooling, the majority were focused on warming.

It is true that a small number of studies in the scientific journals during the seventies focused on the potential for global cooling. Some theorized that the cooling effect from aerosol particulates produced by the industrial activities of mankind would overwhelm the warming caused by greenhouse gas contributions of those same activities. As that decade was at the end of close to 30 years of cooling temperatures, this subset of studies received notable media attention. However, a recent review of the scientific literature of the time found that the majority of studies related to climate were concerned with global warming even then.

A: The greenhouse effect is a property of the atmosphere that traps heat near the Earth’s surface.

As light from the Sun reaches the Earth’s surface, some of it is reflected back into space. However, much of it is absorbed by the Earth’s surface and atmosphere. This absorbed energy can then be radiated back toward space as infrared heat. Certain gases in Earth’s atmosphere, called greenhouse gases, trap some of this heat before it escapes to space, keeping it near the Earth’s surface. This natural effect of Earth’s atmosphere keeps our planet warm.

The greenhouse effect is responsible for retaining the energy received from the Sun and keeping it close to the planet’s surface. Without it, heat would be lost very quickly to space, making daytime and nighttime temperatures vary by several hundred degrees. Take for example our moon. Without an atmospheric greenhouse effect to moderate its temperature, the light and dark sides of the moon vary by 500 degrees Fahrenheit with temperatures much hotter and much colder than any place here on Earth.

A: No. Solar irradiance has slightly declined for several decades while temperatures have risen.

The amount of energy received from the Sun varies by the Earth’s surface moving closer to or farther away from the Sun via changes in the planet’s orbit, direct increases or decreases in solar output, or both. Orbital shifts, and their related climate impacts, occur over thousands of years, much too slow to account for current warming. Solar activity has been monitored in detail via satellite for over 30 years, and average solar irradiance has shown a slight decline, also making it unable to account for warming over that time.

A: Not necessarily, but a steady Sun points to local climate factors to explain any changes.

Satellite observations in the early 2000’s revealed that the ice cap on Mars’ southern pole was receding as a result of warming in that region. However, it is unsupportable to extrapolate from this regional effect to conclude warming of the entire planet and from there attribute warming on both Mars and Earth to a common cause, namely the Sun. This common attribution of solar impacts is also negated by direct satellite observations of solar activities over the last 30 years which have revealed no increase in solar irradiance. Regional changes on Mars are a result of variations in local conditions.

A: Of course, but human activities are a new climate influence that adds to natural factors.

In the same manner that you can’t conclude all car accidents have the same cause, climatic temperature shifts now and throughout history have had different causes. Human activities have now introduced a new influence on the climate never before seen in the 4.5 billion year history of the planet. Our actions have changed the fundamental compositions of the Earth’s atmosphere and oceans, and these changes must be considered alongside natural factors when examining changes in Earth’s climate.

Q: How do we know an increased greenhouse effect is causing the Earth to get warmer?

A: Measurements indicate less energy is escaping to space than is being received from the Sun.

All of the energy received from the Sun must eventually return to space. This is referred to as the Earth’s energy balance. Satellites can directly measure the amount of energy being received from the Sun and the amount of energy escaping from Earth back to space. Continual measurements from multiple satellites indicate that, right now, less energy is escaping to space than is being received from the Sun. More specific measurements show that the energy being retained corresponds to the atmospheric gases that provide the Earth’s greenhouse effect. This additional energy in the Earth system causes the planet to warm. Additional warming will continue until the energy balance is restored.

A: Less than 3%, but the exact amount varies over time and based on location.

The two gases that make up more than 99% of the Earth’s dry atmosphere, oxygen and nitrogen, are not greenhouse gases. So the greenhouse effect, which keeps our planet liveable, depends on a very small percentage of the atmosphere above us. The natural greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Water vapor is the most prominent greenhouse gas, but the amount of water vapor in the air varies greatly from place to place and over time. The amount of water vapor in the air depends on temperature and can vary from 0% of the atmosphere (think desert) to 4% (think jungle). Carbon dioxide, the primary long-lived greenhouse gas, currently makes up about 0.04% of the atmosphere. The other greenhouse gases make up even smaller percentages.

A: Yes, but its very short time in the atmosphere relegates its influence to a feedback effect.

Water vapor, as a greenhouse gas, is very effective at absorbing radiated heat, much more effective than carbon dioxide. However, water vapor only remains airborne for a very short period of time, only a few days, before it precipitates back out, which is why the atmospheric concentration of water vapor varies so much from place to place, over time, and with height in the atmosphere. This very short atmospheric lifetime make it impossible for water vapor to drive long-term climate changes on its own. The amount of water vapor the air can hold is a factor of air temperature. Warmer air can hold more water vapor than can colder air. So, if air temperature increases as a result of increases in solar activity or increases in long-lived greenhouses gases like CO2, which can remain airborne for more than 100 years, the air can hold more water vapor. This additional water vapor absorbs more heat and drives temperatures even higher in a feedback effect.

Q: Don’t increases in temperature lead to increases in CO2, not the other way around?

A: Yes and no. Increases in one lead to increases in the other, no matter which rises first.

Prior to the the industrial age, changes in atmospheric CO2 were largely the result of other preceding natural changes. For instance, changes in the planet’s orbit could increase the amount of sunlight received by the planet’s surface which would increase global temperatures. These increased temperatures would, in turn, cause more CO2 to be released by the oceans and by melting ice. As a greenhouse gas, these newly elevated CO2 levels would trap additional heat near the Earth’s surface, resulting in additional warming in a feedback effect. As a feedback effect, these elevated CO2 levels would lag behind the initiating change but then subsequently contribute to its continued rise. Now industrial activities are rapidly elevating these atmospheric levels with no preceding natural change needed. As a greenhouse gas, these elevated levels must necessarily result in atmospheric warming.

Q: How do we know human activities are driving up the level of CO2 in the atmosphere?

A: For at least 800,000 years, atmospheric CO2 has never risen above 300 ppm. It’s now at 392 ppm.

Scientists can analyze air bubbles in ice cores drilled in ancient glaciers in Greenland and Antarctica to determine the composition of the atmosphere in Earth’s distant past. These analyses show that atmospheric CO2 has never risen above 300 parts per million (ppm) for at least the last 800,000 years and likely the last several million years. Since the onset of the Industrial Revolution atmospheric CO2 has been steadily rising. It is now at about 392 parts per million and continuing to climb as emissions from the burning of fossil fuels continue to increase. We are now burning fossil fuels a million times faster than they were generated by nature.

Q: Aren’t human emissions of CO2 a fraction of those from natural sources?

A: Yes, but natural sources have been counterbalanced by natural sinks over thousands of years.

Imagine a bathtub with a faucet and a drain. Over time the amount of water coming from the faucet changes. To keep the water in the tub at the same level, you have to open or close the drain further to adapt. In the same manner, natural sinks of carbon dioxide adapt to counterbalance changes in natural emissions. These changes generally happen over tens of thousands of years. Now, the industrial activities of mankind have opened the faucet further, adding water much faster than the drain can adapt. As a result, the water level in the tub is rising as greenhouse gases accumulate in our atmosphere and the heat content increases. It is the human contributions of these gases that are throwing off the global balance.

A: No. Human activities release 100 times more CO2 than all of the Earth’s volcanoes combined.

When they erupt volcanoes emit large amounts of ash, aerosols, and volcanic gases into the atmosphere, including CO2. As a greenhouse gas, this additional CO2 can promote warming, but the primary climatic impact from volcanoes comes when aerosol particulates reach the upper atmosphere, or stratosphere. As these aerosols are spread by winds around the world, they can reflect sunlight back into space and promote cooling conditions. But this is a short-lived effect that lasts about 1 to 2 years at most and ends when these particulates fall out of the atmosphere. As for CO2, the U.S. Geological Survey, backed by several studies, has concluded that human activities release more than 100 times the amount of CO2 annually than all of the Earth’s volcanoes on land and underwater combined. If volcanoes truly emitted more CO2 than mankind, you would expect to see large spikes in atmospheric CO2 each time a large volcano erupts. Looking at measurements of atmospheric CO2 over the last 100 years, this has not happened.

A: Not necessarily. High levels of CO2 must be considered with other factors in a warming world.

As plants utilize CO2 in their growth processes, gardeners often pump excess CO2 into greenhouses to encourage enhanced growth. However, greenhouses are very controlled environments beyond simply CO2 levels. Outside of the greenhouses, excess CO2 must be considered alongside other environmental factors. Increased temperatures along with shifting precipitation patterns can result in regional flooding in some areas while promoting widespread drought and expanded desertification in others. These same conditions can result in the proliferation of damaging insect populations such as bark beetles and invasive plant species which can alter the fundamental operations of local ecosystems. Growth response to excess CO2 also varies by plant species and is mitigated by other local conditions such as the availability of nutrients in the soil. This variable response can also lead to much higher pollination rates, resulting in adverse health impacts.

A: Any change at an abnormally high rate or to an abnormally high level can potentially be pollution.

Anyone who has ever processed film from an older camera can attest to the damage simple light can do when introduced during the developing process. This is called light pollution. Similarly in nature there are many ecosystems that have evolved over millions of years under conditions of little to no light. Rapidly introducing large amounts of lights in a sustained way to such environments can have profoundly negative impacts. The plants and animals living under such conditions simply don’t have sufficient time to adapt. This is also true for atmospheric CO2. While it is true that atmospheric levels of CO2 have been much higher in Earth’s very distant past, ecosystems had thousands of years to adapt to such levels. At present, we are changing the atmospheric concentration of CO2 much faster than natural systems can react and adapt, resulting in a polluting impact.

A: Deforestation removes a vital natural method of removing CO2 from the atmosphere.

As we all learned in school, plants use CO2 during photosynthesis and return oxygen to the atmosphere. Through this process the world’s forests remove massive amounts of CO2 from the atmosphere. However, at present, the activities of mankind are simultaneously adding large amounts of CO2 to the atmosphere through the burning of fossil fuels while also eliminating a vital method of absorbing this CO2 from the atmosphere by cutting down immense tracts of forested land. As a result, deforestation is the 2nd largest human contributor to CO2 emissions behind fossil fuel combustion.

Q: If we can’t predict the weather next week, how can we predict the climate in a hundred years?

A: Longer term climate trends can be more predictable than short-term weather variations.

While I may not be able to tell you with precision whether or not it will rain in a particular area a month from now, I can tell you how much rain is typical for that area in that month historically. The former is weather. The latter is climate. Climate is based on long-term trends, generally of 30 years or longer, for a particular region, whereas weather is day-to-day variability which can be much more difficult to predict. Often it’s said, “Climate is what you expect. Weather is what you get.” Or, as one youth put it, “Weather determines what clothes you wear. Climate determines what clothes you buy.” The irony is that, as human activities fundamentally alter the compositions of the atmosphere and the oceans in unprecedented ways, the climate is becoming much less predictable than it is now.

If you plot the temperature of a region over a period of time, you will get a bell-shaped curve with that region’s average temperature at the bell’s highest point. When you increase that average temperature, the entire bell curve shifts with it. As a result, you would expect to see fewer very cold events, more very warm events, and the introduction of extremely warm events. Also, as you raise the temperature, you add more energy to the atmosphere. The climate system moves and dissipates energy through storm systems. With more energy available, storm systems have the potential to become more extreme. This does not mean that every storm will be an extreme one, it simply increases the odds.

The massive size (one-and-a-half times larger than the continental U.S.) and extreme conditions (the coldest and windiest on the planet) around Antarctica make it exceedingly difficult to state definitively whether or not the continent is gaining or losing ice mass overall. Recent gravitational studies using NASA satellites have concluded that Antarctica is losing ice on the whole and at an accelerating rate. Regardless of the conclusion reached regarding ice mass, it is clear that Antarctica is changing based on its responses to a warmer world. Ice sheets, which float on underlying water, are losing ice around the entire Antarctic perimeter. Precipitation in East Antarctica, the largest desert in the world, is increasing, as is sea ice surrounding the continent. Meanwhile the continent itself is becoming greener as warmer temperatures enable new vegetation to take root.

A: Feedbacks are natural responses that can either reinforce warming or oppose it.

The environment has many natural responses to warming conditions. Many of these responses can reinforce that warming resulting in additional warming. These reinforcing responses are called positive feedbacks. As highly reflective ice melts, it is replaced by very dark ocean water which absorbs more heat resulting in additional warming. This is an example of a positive feedback. Other natural responses oppose the warming conditions resulting in less warming. These opposing responses are called negative feedbacks. As more plants grow with higher concentrations of CO2 in the atmosphere, this additional plant life removes more CO2 from the atmosphere. This is an example of a negative feedback. The combined magnitude of these natural feedbacks determines the total impact of a change in the climate system.

A: Tipping points are points beyond which large-scale change is self-sustaining and inevitable.

When you walk into a room and flick the light switch on, you have just caused a tipping point to be crossed. Once you exert just enough force, the switch flips from the OFF position to the ON position. The world is full of tipping points. Snow can accumulate on a mountainside for a long period of time until just enough piles up to trigger an avalanche. Cracks can emerge and grow on a dam over a period of time until they reach a point of failure when the dam breaks. The climate system has tipping points as well. It is these tipping points that have enabled our planet historically to alternate between frigid ice ages and warm interglacial periods. Conditions driven by feedback effects change just enough to shift the planetary climate system into a new state. As human activities continue to fundamentally alter the composition of Earth’s atmosphere, the odds of the climate system crossing one or more tipping points grows. Once a tipping point is crossed, large-scale changes become self-sufficient and can be exceedingly difficult or impossible to reverse.

CFL bulbs do contain a very small amount of mercury, which is a toxic substance. Almost all (more than 95%) contain less than 10 mg with 2/3 containing less than 5 mg as of 2004. By comparison, the majority of standard linear fluorescent bulbs which have been in use in homes, schools, and workplaces for decades, contain at least 5 mg of mercury with 40% containing more than 10 mg ranging all the way up to 100 mg per bulb. Many older home thermostats still in use can contain upwards of 4,500 mg of mercury, the equivalent of 900 CFL bulbs. Essentially, CFL bulbs should be handled responsibly like many other potentially dangerous, but useful, items around the home including cleaning products, medications, pesticides, and firearms.

A: They have and continue to do so within the peer reviewed scientific journals.

The public debate format can be a productive forum for presenting and discussing matters related to policy decisions, and this holds true for policy matters related to climate change. However, the scientific foundation for such discussions is based on evidence gathered through detailed observation and analysis which is documented, presented, and discussed within the peer-reviewed journals of the scientific community.